Laser welding is a metal joining process in which a laser beam is directed at a metal workpiece stack-up to provide a concentrated heat source capable of effectuating a weld joint between the component metal workpieces. In general, two or more metal workpieces are first aligned and stacked relative to one another such that their faying surfaces overlap and confront at an intended welding site. A laser beam is then directed at a top surface of the workpiece stack-up. The heat generated from the absorption of energy from the laser beam initiates melting of the metal workpieces and establishes a molten weld pool within the workpiece stack-up. The molten weld pool penetrates through the metal workpiece impinged by the laser beam and into the underlying metal workpiece or workpieces. When the laser beam has a high enough power density, a keyhole is created within the molten weld pool directly underneath the laser beam (a process known as “keyhole welding”). A keyhole is a column of vaporized metal derived from the metal workpieces within the workpiece stack-up and may include plasma.
The keyhole provides a conduit for energy absorption deeper into workpiece stack-up which, in turn, facilitates deeper penetration of the molten weld pool and a narrower weld pool profile. As such, the keyhole is normally controlled to penetrate into the workpiece stack-up across each faying interface, either fully or partially through the bottom-most metal workpiece. The keyhole is typically created in very short order—on the order of milliseconds—once the laser beam impinges the top surface of the workpiece stack-up. After the keyhole is formed and stable, the laser beam is moved a short distance along a weld path. Such movement of the laser beam leaves behind a trail of molten workpiece material in the wake of the corresponding travel path of the keyhole and molten weld pool. This penetrating trail of molten workpiece material cools and solidifies in the same direction as the forward movement of the laser beam to provide a laser weld joint that fusion welds the workpieces together.
Many industries use laser welding as part of their manufacturing practice including the automotive, aviation, maritime, railway, and building construction industries, among others. Laser welding is an attractive joining process because it requires only single side access, can be practiced with reduced flange widths, and results in a relatively small heat-affected zone that minimizes thermal distortion. In the automotive industry, for example, laser welding can be used—and indeed it frequently is used—to join together metal workpieces during manufacture of the body-in-white (BIW) as well as finished parts that are installed on the BIW prior to painting. Some specific instances where laser welding may be used include the construction and attachment of load-bearing body structures within the BIW such as rail structures, rockers, A-, B-, and C-pillars, and underbody cross-members. Other specific instances where laser welding may also be used include non-load-bearing attachments within the BIW, such as the attachment of a roof to a side panel, and to join overlying flanges encountered in the construction of the doors, hood, and decklid. Both full and partial penetration laser welds may be employed when needed.
In an effort to incorporate lighter weight materials into a motor vehicle, and thus improve fuel economy, there has been a push to incorporate aluminum alloys into the vehicle platform wherever practical. A stack-up of overlapping aluminum alloy workpieces can certainly be joined in spot or seam fashion by laser welding. In some instances, however, particularly when at least one of the aluminum alloy workpieces is composed of a 5000 or 6000 series aluminum alloy, the laser weld joint may experience hot cracking which, in turn, can keep the joint from attaining its maximum strength. Hot cracking occurs during solidification of the molten aluminum alloy material produced by the laser beam when strain at the fusion boundary of the weld zone exceeds material ductility. This strain is believed to cause the liquid film between grains to break and form a cavity where insufficient liquid metal is available to backfill the cavity, thus inducing out-of-plane deformation of one or more of the overlapping aluminum alloy workpieces. When such deformation occurs, the resultant tensile strain imparted to the solidifying workpiece material causes a crack to propagate from a root(s) of the weld joint upwards through the weld joint to the surface of the workpiece stack-up acted on by the laser beam. A laser welding method that includes provisions to eliminate or at least reduce the chance of hot cracking without using a filler wire to add material to the molten aluminum alloy weld pool is therefore needed.